The synthetic and mechanistic aspects of the photochemistry of 1,3- s-cis- and 1,3-s-trans-dienes in steroids, terpenes, and related natural products is being studied. The photochemistry of the 1,3,5-hexatriene chromophore contained in the same series of natural products also is being investigated. The photochemistry of acyclic Beta, gamma-unsaturated ketones is being studied and the results obtained being utilized to understand better the photochemical reactivity of this chromophoric grouping in natural products. A structure-reactivity relationship is being investigated in (2 plus 2) photocycloaddition reactions in order to develop this reaction as a synthetic procedure for the synthesis of natural products.